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www.dmu.dk Fig. 1. cold- (a) and hot-water (b) extractable organic C Recycling of bioenergy waste-stream materials to soil in organic farming systems A Johansen 1, H Hauggaard-Nielsen 2, P Ambus 2, MS Carter 2 1) National Environmental Research Institute, University of Aarhus; 2) Risø National Laboratory, Technical University of Denmark 17 BIOCONCENS – a 4-year interdisciplinary projects addressing production and use of energy in organic agriculture (OA) in order to reduce the reliance on non-renewable fossil fuels and minimize greenhouse gas (GHG) emissions, while at the same time considering soil fertility and socio-economical aspects. In the BioConcens (Biomass and bioenergy-production in organic agriculture) project one important objective is to evaluate how waste-stream materials from bioethanol and biogas production affect the soil fertility when recycled to the field as fertiliser. Applying the residue materials from bioenergy production, there may be a risk that the decreased content of organic carbon and the readily available mineral nutrients will have a negative effect on soil fertility parameters - like the soil microbial community, soil structure and content of organic carbon. This poster presents data from a incubation experiment where residues from biogas production were incorporated into soil and compared with raw manure and a green (clover-grass) manure with regard to soil quality parameters that are crucial in organic farming systems. Acknowledgements The project is linked to the Danish Research Centre for Organic Farming and funded under the research programme: Research in Organic Food and Farming, International Research Co- operation and Organic Integrity (DARCOF III 2005-2010) Materials & Methods Soil sampled from an organically managed field was amended with raw cattle manure, de-gassed manure, de-gassed mixture of manure and maize, de-gassed mixture of manure and clover-grass or fresh clover-grass (green manure). The materials were incorporated homogeneously into separate soil volumes, which were sampled destructively at 0, 1, 3 and 9 days after incorporation. Assays: 1) Labile soil organic C and mineral N pools (water extractable), 2) Greenhouse-gas emissions (N 2 O and CO 2 ), 3) Soil mineral N concentration, 4) Microbial genetic (PLFA) and functional (MicroResp) diversity in the soil. Fig. 2. CO 2 (a) and N 2 O (b) emissions Fig. 3. Soil NO 3 (a) and NH 4 (b) concentration Fig. 4. Genetic (a) and functional (b) diversity of microb. community (PC-analysis; incubation day inside symbols; symbols as other figs). Results Labile org. C content highest with clover-grass applied (Fig. 1). Clover-grass also caused the highest emissions of CO 2 and N 2 O (Fig. 2). Available mineral N was enhanced with the de-gassed materials applied, while clover-grass caused complete N immobilization (Fig. 3). Microbial diversity was indeed changed by the different treatment, although this effect was apparently only temporary (Fig. 4). Conclusions In the current experimental setup, residues after bio-gasification seems suited for fertilizer usage and not much different from application of raw cattle manure. However, the long-term effects on soil organic matter content needs to be further clarified. Application of clover-grass to the soil caused a significant loss of C and N due to gaseous emissions. This specific situation was, however, not observed in a similar parallel field experiment. corresp. Anders Johansen, ajo@dmu.dk www.risoe.dk